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Petersen Graph

Suppose you have a set with 5 elements. There are 10 ways to choose a 2-element subset. Form a graph with these 10 choices as vertices, and with two vertices connected by an edge precisely when the corresponding subsets are disjoint. You get the graph shown here, called the Petersen graph.

• Japheth Wood, Proof without words: the automorphism group of the Petersen graph is isomorphic to \(\mathrm{S}_5\), Mathematics Magazine89 (October 2016), 267.

As the title indicates, it’s easy to use this picture to determine the symmetry group of the Petersen graph.

The Petersen graph is reputed to be a counterexample to many conjectures about graph theory, and it shows up in many places. We have described it as an example of a ‘Kneser graph’. The Kneser graph \(KG_{n,k}\) is the graph whose vertices correspond to the \(k\)-element subsets of an \(n\)-element set, where two vertices are connected by an edge if and only if the two corresponding subsets are disjoint.

We can also get the Petersen graph by taking a regular dodecahedron and identifying antipodal vertices and edges.

Or, take the complete graph on 5 vertices, \(K_5\), and form a new graph with the edges of \(K_5\) as vertices, with two of these vertices connected by an edge if the corresponding edges in \(K_5\) do not share a vertex. The result is the Petersen graph! We say the Petersen graph is the complement of the line graph of \(K_5\).

The Petersen graph also shows up when you consider all possible phylogenetic trees that could explain how some set of species arose from a common ancestor. These are binary trees with a fixed number of leaves where each edge is labelled by a time in $[0,\infty)$. The space of all such trees is contractible, but nonetheless topologically interesting. The space of phylogenetic trees with 4 leaves is the cartesian product of the cone on the Petersen graph and $[0,\infty)^5$. For pictures illustrating this, see:

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